Chemical Geology, 57 (1986) 269--288 269 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

[3]

BEHAVIOUR OF TRACE ELEMENTS DURING MAGMATIC PROCESSES IN THE CRUST: APPLICATION

TO ACIDIC VOLCANIC ROCKS OF TUSCANY (ITALY)

A. G I R A U D 1 , C. D U P U Y 1 a n d J. D O S T A L 2

1 Centre GOologique et GOophysique, UniversitO des Sciences et Techniques du Languedoc (USTL), 34060 Montpellier COdex (France)

2 Department of Geology, Saint Mary's University, Halifax, N.S. B3H 3C3 (Canada)

(Received May 1, 1986; accepted for publication June 19, 1986)

A b s t r a c t

Giraud, A., Dupuy, C. and Dostal, J., 1986. Behaviour of trace elements during magmatic processes in the crust: Application to acidic volcanic rocks of Tuscany (Italy). Chem. Geol., 57: 269--288.

Plio-Quaternary acidic rock suites from three volcanic centres of Tuscany (Italy) - - San Vincenzo, Roccastrada and Monte Amiata -- are composed predominantly of rhyolitic and rhyodacit ic ignimbrites. The rocks are characterized by distinct enrichments, particularly of K and Rb, and display considerable composit ional differences among the various volcanic centres. The volcanic rocks of Roccastrada and most of S. Vincenzo were probably generated by partial melting of metapeli t ic rocks from the Paleozoic basement of Tuscany, leaving a residue composed of biotite, cordierite and quartz and some- times plagioclase or garnet. The enrichment of K and related elements can be at tr ibuted to selective contaminat ion by fluids. The other volcanic rocks including trachytes of Mt. Amiata have also been affected by interaction with an upper-mantle component , probably K-rich basaltic rocks. A similar petrogenetic model can be applied to the granodiorite pluton of Mt. Capanne (Elba) which belongs to the same magmatic cycle. Since the variation trends observed in the Tuscany volcanic rocks are similar to those reported from many acidic plutons, it appears that crustal anatexis accompanied by interaction with upper-mantle components has frequently p layed an important role in the genesis of the granitic rocks.

1. I n t r o d u c t i o n

S i n c e t h e p i o n e e r i n g w o r k o f G a s t ( 1 9 6 8 ) , n u m e r o u s g e o c h e m i c a l s t u d i e s h a v e f o c u s e d o n p a r t i a l m e l t i n g o f t h e u p p e r m a n t l e a n d t h e b e h a v i o u r o f t r a c e e l e m e n t s d u r i n g t h i s p r o c e s s is r e l a t i v e l y w e l l - u n d e r s t o o d . O n t h e o t h e r h a n d , t r a c e - e l e m e n t g e o c h e m i s t r y o f c r u s t a l a n a t e x i s is o n l y p o o r l y k n o w n , in

p a r t p r o b a b l y b e c a u s e o f t h e c o m p l e x i t i e s o f t h e s u p e r i m p o s e d e f f e c t s o f v a r i o u s p r o c e s s e s such as m a n t l e - - c r u s t i n t e r a c t i o n s w h i c h h i n d e r t h e i n t e r p r e t a t i o n o f t h e d a t a . F u r t h e r m o r e , m o s t o f t h e s t u d i e s d e a l w i t h g r a n i t i c r o c k s ( H a n s o n , 1 9 7 8 ; M c C a r t h y a n d K a b l e , 1 9 7 8 ; Mi l l e r a n d M i t t l e f e h l d t , 1 9 7 9 ) w h i c h a l so u n d e r w e n t l a t e p o s t - m a g m a t i c a l t e r a t i o n .

0009-2541/86/$ 03.50 © 1986 Elsevier Science Publishers B.V.

270

This paper concerns trace elements in a well-studied magmatic suite from Tuscany (Italy). Crustal melting played a dominant role in its genesis (Marinelli, 1961; H.P. Taylor and Turi, 1976) although some; interaction with mantle-derived magma has been postulated in the southern part of the region (Hawkesworth and Vollmer, 1979; Poll et al., 1984; van Bergen, 1984, 1985). This s tudy at tempts to quantify the main petrogenetic process and to evaluate the behaviour of trace elements during the various events in the genesis of these rocks.

2. Geological notes

The study deals with acidic volcanic suites from three volcanic centres of Tuscany: San Vincenzo, Roccastrada and Monte Amiata (Fig. 1). In addition, the granodiorite of Monte Capanne (Elba) which belongs to the same magmatic cycle (Marinelli, 1961) has also been investigated in order to evaluate whether the slow cooling modified its chemical composit ion relative to that of ignimbrites. The genetic relation- ship between the plutonic and volcanic rocks is also discussed.

Detailed volcanological, petrographic and

1

~11 V o h : a n i c s ,

N P,s,l /~l , ~ J ' 43 ~' ' ' £ . . . . . . . [ , [ : : _ q R . . . . . . . . . P . . . . . . . . .

l I vo ' r l o ~

S. V i n c e n z o ° S m ' l ~

~! k ( ~ It~ R o c c a s t r a d a E I b a r ~ * M ' A m i a t a

' , Grosseto ~ , ,el: "

o .....

/

.... ,~ !! ~,~-

I 0 50 100 km

/ Fig. 1. Map showing the location of the studied vol- canic eentres.

geochemical descriptions of these rocks have been presented in several papers (e.g., Barberi et al., 1971; Van Bergen, 1985). The volcanic complex of S. Vincenzo spreads over an area of ~ 1 0 k m ~ in the Campigliese province while that of Roccastrada is located 35kin southeast in the Grosseto province where acidic volcanic rocks form isolated outcrops stretching over 30kin 2. The age of these complexes is respectively 4.7 and 2 .3Ma (Borsi et al., 1967). In both areas, volcanic rocks occur as lava flows, domes and ignimbrites with rhyolitic and rhyodacitic composition. The rocks are highly porphyrit ic and the main phenocryst phases are plagioclase, sanidine, quartz, biotite and subordinate cordierite.

The volcanic complex of Mt. Amiata in southern Tuscany is located 25 km north of Vulsini, the nor thernmost volcanic centre of the Roman magmatic province. It is the youngest volcanic occurrence in the Tuscan magmatic province ( 0.18--0.29 Ma old; Bigazzi et al., 1981). According to Van Bergen (1985), the complex is composed of four units: (1) early acidic lavas and ignim- brites; (2) acidic dome lavas; (3) minor late acidic lavas; and (4) a small volume of the youngest intermediate lavas usually with trachytic composition. The acidic lavas are porphyrit ic latites and rhyodacites with phenocrysts of sanidine, plagioclase, or thopyroxene, cl inopyroxene and biotite. The intermediate lavas are hybrid rocks which contain phenocrysts from rhyodacites and minor amounts of olivine and cl inopyroxene from basaltic magma (Van Bergen, 1985).

All the volcanic suites contain inclusions of igneous and metasedimentary rocks (Van Bergen, 1985), which are particularly abundant in the Mr. Amiata complex. The metasedimentary xenoliths probably played a role during the genesis of the acidic magma and were derived from the pre-Mesozoic basement of Tuscany (Van Bergen, 1985).

The granodiorite of Mt. Capanne which

271

crops out as a pluton in the western part of island of Elba is medium-grained rock composed of zoned plagioclase, quartz, orthoclase, biotite and accessories (tourmaline, apatite, zircon, orthite, sphene and magnetite) and contains metasedimentary and igneous inclusions. The U--Pb age of zircon is 6.2 Ma (Juteau et al., 1984).

3. Analytical methods

Twenty-five whole-rock and ten const i tuent groundmass/glass samples were selected from a suite studied by Dupuy (1970). In addition, 18 mineral phases were separated from these rocks by conventional magnetic and heavy-liquid techniques fol- lowed by hand-picking under a binocular microscope. The purity of the mineral sepa- rates was better than 99%. In all these samples, the major elements and Li, Rb, Sr, V, Cr, Ni, Co, Cu and Zn were analyzed by atomic absorption. Ba, Pb, Ga, Zr, Y and Nb were determined by X-ray fluorescence and rare- earth elements (REE), Hf, Th, U and Sc were analyzed by instrumental neutron activation. The precision and accuracy of the analytical procedures were presented by Dostal et al. (1983). Ten samples of ignimbrites from S. Vincenzo and Roccastrada were analyzed for Sr isotopes at Laboratoire de Geologie, Universite de Clermont-Ferrand by P. Vidal. The precision of s7 Sr/S6 Sr values is + 0.00004 (2o) calculated from replicate analyses of the standard NSS 9 8 7 (= 0.71024 + 0.00004). The details on the isotope tech- niques were given by Vidal et al. (1984).

4. Geochemical data

4 . 1 . W h o l e - r o c k s

The whole-rock geochemistry, particularly of major elements, for these rocks has been reported elsewhere (Dupuy, 1970; Dupuy and All~gre, 1972) and thus the discussion is focused mainly on the distribution of trace elements.

All the Tuscan rocks (Table I) have peraluminous character with corundum in their norms. In addition to major elements (Fig. 2), the trace-element distribution is distinct in the four studied localities (Table II). The acidic volcanic rocks of Mt. Amiata have the highest content of Ba, Sr, Pb, Hf, Th, Nb, Y, light REE (LREE) and transition elements whereas the rocks of Roccastrada have the lowest abundances of these elements. The S. Vincenzo volcanic rocks are inter- mediate between these two localities. The composition of grandodiorite of Mt. Capanne closely resembles some ignimbrites from S. Vincenzo with the exception of Li and heavy REE (HREE) which are lower in the volcanic rocks.

O

t 2000 ba~ 46 / X

~ , o ,\ / S ' / \

Ab / '/ "\'\ ~ O r

Fig. 2. Normative compositions of the rocks from Tuscany projected on the quartzllbite--orthoclase plane of the Q--Ab--Or--An--H~O system (Winkler, 1979). Cotectic lines corresponding to various Ab/An ratios are shown. S o l i d s y m b o l s = whole-rocks; o p e n s y m b o l s = groundmass/glass; squares = Roccastrade; t r iangles = S. Vincenzo; circ les = Mr. Amiata; crosses

= Mt. Capanne granodiorite.

In each locality, the trace elements display only limited variations. The rocks of Mt. Amiata are particularly homogeneous with compositional variations within the range of analytical uncertainty (~ 2--3% for Rb, Sr, Ba; ~ 5% for REE). In the other suites, there are small but distinct differences between various volcanic units which represent different eruption cycles. In particular, the youngest trachytic lava flows have the lowest SiO2 and relatively high Fe,

272

TABLE I

Major-element composit ion of whole-rocks

Mt. Amiata S. Vincenzo

Group I

Ref. A1 A3 A 7 A8 AIO A 1 9 A 2 5 A 3 2 A33 V1 V8 V15

SiO2 (wt.%) 66.09 66.15 67.55 66.05 65.84 65.33 63.33 63.74 60.31 70.16 69.37 68.76 A1203 (wt.%) 15.27 15.71 15.6 15.53 15.69 15.76 15.68 16.04 15.56 15.10 14.82 14.94 Fe2Os (wt.%) 3.43 3.29 3.38 3.33 3.40 3.84 4.42 3.93 4.94 2.07 2.50 2.59 MnO (wt.%) 0.05 0.05 0.05 0.05 0.05 0.06 0.07 0.06 0.08 0.03 0.03 0.03 MgO (wt.%) 1.31 1.29 1.34 1.29 1.36 1.62 2.35 1.76 3.15 0.66 0.95 0.91 CaO (wt.%) 2.82 2.81 3.01 3.00 3.05 2.81 4.38 3.42 5.42 1.54 2.02 1.96 Na20 (wt.%) 2.27 2.30 2.29 2.31 2.30 2.15 2.11 2.16 2.02 3.11 2.94 2.86 K~O (wt.%) 5.67 5.82 5.71 5.75 5.86 5.69 5.44 5.61 5.46 4.53 4.67 4.63 TiO2 (wt.%) 0.5] 0.51 0.52 0.51 0.53 0.52 0.61 0.57 0.63 0.30 0.35 0.38 P2Os (wt.%} 0.17 0.17 0.18 0.17 0.17 0.19 0.19 0.19 0.23 0.14 0.17 0.20 H20 ÷ (wt.%) 1.27 0.93 0.49 1.00 0.71 1.21 1.22 1.85 1.02 2.92 1.76 1.93 H 2 0 - (wt.%) 0.23 0.09 0.15 0.07 0.09 0.55 0.25 0.30 0.19 0.21 0.16 0.21

?: 99.09 99.12 100.28 99.06 99.05 99.73 100.05 99.63 99.01 100.77 99.74 99.40

A 1 - - A I O = ignimbritic lava; A19, A25 = lava dome; A32~ A33 = trachytes; V 1 - - V 1 2 = ignimbritic lava; R 3 - - R 1 9 = ignimbritic lava; E8--E15 = granodiorite.

Mg and K, displaying certain similarities with lamprophyres (Van Bergen, 1985). The trachytic rocks also have high contents of LREE, Sr, Ba, Zr and transition elements. In fact, in spite of the small composit ional variation range, the volcanic rocks of S. Vincenzo can be subdivided into two groups (Tables I and II). Group I with lower Si, Li and STSr/S6Sr ratio has higher LREE, Sr and Ba contents.

The chondrite-normalized g E E patterns of the acidic rocks from Tuscany and Elba (Fig. 3) display LREE enrichment and a negative Eu anomaly which is particularly pronounced in the volcanic rocks of Roccastrada. The La/Yb ratio ranges from 10 in the rocks of Roccastrada to 30 in the S. Vincenzo volcanic rocks. The patterns of the rocks from S. Vincenzo and Roccastrada have fractionated HREE whereas the volcanics of Mt. Amiata and granodiorites of Mt. Capanne show relatively fiat patterns for HREE. In general, the REE patterns of acidic volcanic rocks from Tuscany are similar to those of other rhyolitic rocks (Mahood,

1981; Cameron and Hanson, 1982) and granites (Mittlefehldt and Miller, 1983; Price, 1983).

In the individual suites, most of the analyzed trace elements are intercorrelated and display regular trends when plot ted against SIO2. There are also some systematic variations when all the rocks are considered together. With the increase of SiO2 from the volcanic rocks of Mt. Amiata which are the lowest in silica, towards the rocks of Roccastrada with the highest SiO 2 content, Ba, Sr, Ti, V, Zr, Hf, Pb and LREE decrease. Such evolutionary trends are common in acidic volcanic (Hildreth, 1979; Mahood, 1981) and plutonic rocks (Price, 1983; Arniaud et al., 1984) and have also been encountered in organic andesitic suites, where they have been interpreted as mixing between two distinct components {Gerlach and Grove, 1982).

The studied samples have composit ions similar to the average upper continental crust (S.R. Taylor and McLennan, 1981) with the exception of the distinct enrich-

273

Group I I

V l l V12

Roccas t rada

R3 R5 R6 R9 R 1 2 R14 R19

Mt. Capanne

E8 E12 E14 E15

67.76 67.9 14.85 14.94

2.86 2.73 0.04 0.04 1.15 1.08 2.34 2.15 2.72 2.90 4.52 4.59 0.41 0.42 0.21 0.19 2.73 1.90 0.34 0.20

99.93 99.09

72.48 73.48 73.95 73.26 73.26 72.33 72.98 13.56 13.23 13.50 13.80 13.61 13.65 13.26

2.01 1.90 1.63 1.76 1.91 2.04 1.76 0.03 0.04 0.02 0.02 0.02 0.03 0.03 0.36 0.32 0.27 0.32 0.34 0.36 0.26 1.02 0.82 0.65 0.84 0.98 1.02 0.72 2.61 2.79 2.42 2.44 2.61 2.63 2.71 4.86 4.69 4.86 5.09 5.00 4.94 4.78 0.24 0.20 0.21 0.24 0.24 0.22 0.19 0.14 0.16 0.12 0.13 0.13 0.14 0.14 1.56 1.56 1.26 1.00 1.13 1.62 2.05 0.10 0.07 0.22 0.22 0.13 0.11 0.22

98.97 99.26 99.11 99.12 99.36 99.09 99.10

67.62 66.94 66.84 68.58 15.64 15.94 ] 5.94 15.36

3.04 3.21 3.19 2.93 0.04 0.05 0.05 0.05 1.39 1.53 1.54 1.28 2.68 2.86 3.05 2.61 3.30 3.40 3.54 3.26 4.15 4.05 4.03 4.03 0.49 0.51 0.50 0.48 0.15 0.15 0.14 0.14 0.59 0.40 0.39 0.38 0.02 0.02 0.02 0.02

99.11 99.06 99.23 99.12

o z

100

(A )

100

I- E: c~

I A Yb Lu

10

La Ce Nd Sm Eu Tb

(C}

La Ce Nd Sm Eu Tb Yb Lu

-- 100 o: o z o I c)

10

( B )

i J I I i I I I La Ce Nd Sm Eu Tb Yb Lu

100

C3 g -

~ 10 - -

( D )

~'Ys %'

i I 1 I i I _ _ I J La Ce Nd Sm Eu Tb Yh Lu

Fig. 3. Chondr i t e -normal ized REE pa t t e rns of whole-rocks (A ---- Roccas t rada ; B = S. Vincenzo; C = Mt. Amiata; D = granodior i te o f Mt. Capanne; dashed field --- sed imenta ry rocks) (Nance and Taylor, 1976.)

274

TABLE II

Trace-element composi t ion of whole-rocks

Mt. Amiata

Ref. A1 A3 A 7 A8 A I O A19 A25 A32 A33

S. Vincenzo

Group I

V1 V8 V15

Li (ppm) 140 147 Rb 390 390 Sr 369 380 Ba 530 585 Sc 8 V 45 47 Cr 25 25 Co 6 Ni 25 24 Cu 4 5 Zn 53 58 La 70.1 Ce 138 Nd 58.7 Sm 10.6 Eu 1.40 Tb 0.77 Yb 2.93 Lu 0.34 Y 33 34 Hf 6.7 Zr 233 232 Nb 16 16 Pb 60 59 Ga 21 23 Th 43 44 U 10 S~Sr/~ Sr

138 146 144 116 119 147 392 391 395 367 357 365 386 391 388 352 465 433 555 555 550 545 585 590

8 8 8 12 10 41 51 60 50 76 57 26 26 33 27 52 37

7 7 7 5 9 22 23 23 25 31 26

8 7 8 8 8 10 54 51 56 54 59 59 72.1 65.4 67.4 74.8 71.7

146 132 136 148 143 59.6 54.3 55.5 63.1 61.3 10.9 10.2 10.4 11.4 11.1

1.46 1.38 1.39 1.69 1.57 0.79 0.76 0.77 0.80 0.74 3.18 2.79 2.94 3.00 2.93 0.36 0.33 0.33 0.36 0.36

33 32 32 30 35 33 6.6 5.5 5.6 7.1 6.1

238 213 211 242 239 250 17 16 16 17 16 16 60 58 57 57 56 63 22 21 21 21 21 24 45 41 42 45 43 44 12 11 11 10 10

107 162 95 132 349 298 313 310 546 107 251 223 660 275 360 400

15 5 6 6 104 28 43 36

83 20 26 28 14 4 5 5 33 15 17 21 14 3 2 71 63 60 69 70.3 28.5 4 5 5 43.4

142 59.4 94.3 90.6 62.9 27.0 39.8 43.2 11.6 6.25 8.13 8.89

1.73 0.95 1.12 1.28 0.78 0.82 0.84 1.01 2.67 1.32 2.01 1.81 0.33 0.14 0.23 0.21

33 16 22 24 5.8 3.0 4.4 4.1

247 107 139 149 15 11 13 13 53 50 46 50 22 25 22 23 39 13 25 20 10 14 11

0.72444 0.71558

ment of K, Li, Rb, Pb, Th and U. The ratios of several incompatible elements such as Y/Yb, Ti/Zr, Rb/Pb, Zr/Hf and Zr/Nb remain relatively constant among all samples and are closely comparable to those of the average crustal values. On the other hand, Rb/Sr and Th/La ratios are higher whereas K/Rb, Ba/Rb, Ba/Pb, Th/U and Zr/La are lower in volcanics of Tuscany compared with the average upper crust.

Several trace elements normalized to the upper-crustal values are plotted in Fig. 4. The shape of the pattern shows that elements which plot in the graph between U and Sm are enriched relative to the crust while those from the section between Nb and Ba are

depleted. The exceptions are rocks of Mt. Amiata where the latter group of elements has abundances close to those of the crust. The patterns in Fig. 4 suggest that the elements of the first group behave like incompatible elements (bulk partition co- efficient, D ~ 1) whereas those of the second group behave like compatible ones (D ~ 1). All the samples display a negative Nb anomaly and the volcanic rocks of Roccastrada and S. Vincenzo also have a negative Th anomaly. The absence of positive anomalies of Sr and Ba in Fig. 4 suggests that these rocks do not contain feldspars as a cumulate phase or xenocrysts. These two elements together with the

2 7 5

Group II

Roccastrada

V l l V 1 2 R3 R 5 R 6 R9 R 1 2 R14

Mt. Capanne

R 1 9 E8 E l 2 E l 4 E l 5

48 64 156 206 185 155 140 296 306 395 447 449 403 382 311 268 68 51 48 66 70 445 420 172 95 110 165 165

7 7 5 5 5 46 48 12 13 13 17 13

10 9 3 3 3

19 3 2 1

70 74 52 53 43 46 46 57.8 46.5 30.5 24.8 29.2

120 94.7 68.1 52.0 61.6 54.3 41 .9 28.8 24.3 28.0 10.7 8.50 6 .80 5.69 6 .30

1.60 1.24 0.55 0 .40 0.55 1.10 0.87 1.12 0,90 1.10 1.98 1.60 3.09 2.38 3.03 0 .22 0.18 0.33 0.28 0.32

24 19 33 30 26 29 33 4.4 4.1 3.8 3.1 3.5

155 150 113 97 95 108 113 13 12 14 14 15 14 14 50 48 45 38 42 44 45 24 23 20 20 21 21 19 23 21 24 19 20 21 22 12 12 17 20 15

0 .71329 0 . 7 1 4 1 9 0 .71799 0 .71971

154 196 221 208 121 230 390 470 283 277 266 288

70 45 200 218 218 196 160 90 350 415 370 315

5 8 8 8 14 12 34 40 38 36

32 32 34 3 7 7 7

18 15 14 3 1

51 48 53 66 68 57 34.1 37.5 41.7 30.2 71.2 73.3 83.7 55.9 34.0 32.4 36.2 25.8

7.19 6 .90 7.40 6 .30 0.53 1.07 1.01 0.98 1.23 0.46 0.50 0.51 3 .25 1.93 1.90 2.10 0.35 0.22 0.22 0.22

32 31 22 22 23 21 3.9 3.6 4.0 3.7

107 89 142 155 151 135 14 16 13 14 13 13 47 42 43 46 51 52 20 22 21 24 24 23 25 19 18 22 27 32 15 13 14 16

0 .71818 0 . 71990

transition trace elements are depleted, particularly in the rocks from Roccastrada, possibly because of the presence of feldspars and Fe--Mg minerals in the residue after partial melting.

4.2. Groundmass

Compared to the whole-rocks, the ground- masses have higher Si and K, and lower P, Ti, Fe, Mg and transition trace elements (Table III). They plot close to the ternary minimum at the Q--Ab--Or diagram (Fig. 2) with the exception of sample A25 which is enriched in K. The volcanic rocks of Roccastrada are higher in SiO2 than constituent groundmasses

due to the presence of ~ 5% of quartz xeno- crysts (Balducci and Leoni, 1981).

The REE patterns of groundmasses and associated whole-rocks are shown in Fig. 5. They all display subparallel patterns but with variable negative Eu anomalies. The patterns of groundmass have a more pronounced Eu depletion. Excluding Eu, the REE abundances in the groundmasses from Mt. Amiata are higher than in the corresponding whole-rocks. The rocks from Roccastrada and S. Vincenzo exhibit a reverse relationship with the ground- masses.

Fig. 6 shows the differences in trace- element composition between groundmasses and whole-rocks. In addition to Eu, Sr, Ba

276

< b~

e.

0

8

~ d d d d ~ d d ~ d

c4

¢o [ o L©

i

e o ¢q co ,--4

o - - ° ° ° ° ° ° ° " "" '

0 oo uo . ~ c,I

277

and transition elements are depleted in the groundmasses while Rb is enriched implying fractionation of feldspars and Fe--Mg minerals. The small depletion of Zr and Hf

8 - A

- " \ V . . / ~ " " ~

r- I G U TTh db I ] d. Sfr. EuTrb~b 4 N'bd, Z' Z= ~cTI, Sl Vr aT ~: Pb La r

e B

4

2

3 c )

D

I I , I [ q ' I I r I I I r I I I T q u T h Rb Pb La Ce Sm Eu T b Y b Y N b Hf Zr Zn Sc Ti Sr V Ba

Fig. 4. Average trace-e lement abundances o f the s tudied rocks normal ized to the upper cont inenta l crust (Tay lor and McLennan, 1 9 8 1 ) . (A = Mt. Amiata and ML Capanne; B = S. V i n c e n z o and Roccastrada; symbols are the same as in Fig. 2 e x c e p t S. V i n c e n z o ; solid triangles = group I; open triangles = group II.)

is consistent with fraetionation of zircon. The groundmasses from Roccastrada and S. Vincenzo (the latter not shown in Fig. 6) are depleted in Th and LREE relative to the

2 0 0 •

1 0 0 i

5 0

1 0

3

3 0 0

2 0 0 •

1 0 0 i r

a

c )

10 .

i

1 L

I t (A)

"=. z?.~

.. '..%'-.?,

l IB)

%.

I I J _ _ L _ l : La Ce N d S m Eu T b Y b L u

Fig. 5. Chondr i te -normal ized REE abundances in the who le -rocks (solid lines) and corresponding ground- masses (dashed lines) (A = S. V i n c e n z o ; B = Mt. Amiata and Roccastrada . )

=o

~o.4

o

~0.1

A8

] I U Th Rb La Ce Nd S m Eu T b Yb Lu Y Pb Nb Hf Zr Ti Sr Ba Co V

Fig. 6. Trace-e lement abundances in the groundmass relative to the whole -rocks in t w o se lected samples from Roccastrada ( R 3 ) and Mt. Amiata (AS) .

278

hos t rocks , suggesting, in a c c o r d a n c e wi th the available pa r t i t ion coe f f i c i en t values (Crecra f t e t al., 1981; M a h o o d and Hi ldre th , 1983) , l imi ted al lanite f r ac t iona t ion . Fig. 6 suggests t h a t U is p r o b a b l y the m o s t i n c o m p a t i b l e e l e m e n t whereas the m o s t c o m p a t i b l e t race e l emen t s are a lkal i -ear th and t rans i t ion e lements . The d i f fe rences in the behav iou r of Th and R E E a m o n g indi- vidual local i t ies re f lec t the d i f fe rences in the na tu re o f accessory phases and poss ib ly also the var ia t ions in the values of the pa r t i t ion coef f ic ien t s o f the pr inc ipa l minera l phases due to the changes in l iquid c o m p o s i t i o n (e.g., Watson, 1976; R y e r s o n and Hess, 1978) .

With the e x c e p t i o n of the g r o u n d m a s s of sample V 1 2 which has a c o m p o s i t i o n similar to the hos t rock , all o the r g r o u n d m a s s

TABLE IV

samples p r o b a b l y r ep re sen t residual l iquids a f t e r c rys ta l l i za t ion o f the p resen t pheno- crysts , a h y p o t h e s i s invoked by D u p u y (1970) and D u p u y and Allggre (1972) .

4 3. Minera l phases

The R E E pa t t e rn s o f all minera ls d isplay an e n r i c h m e n t o f L R E E wi th a posi t ive Eu a n o m a l y fo r plagioclase and sanidine and a negat ive Eu a n o m a l y for b io t i te and cordier i te . Cord ier i te has a p a t t e r n similar to b io t i te bu t the a b u n d a n c e s of R E E and t rans i t ion e l emen t s are lower by a f a c t o r o f a t least 2.

Biot i te exhib i t s a large range o f t race- e l e m e n t a b u n d a n c e s (Table IV) which c a n n o t be exp la ined on ly by var ia t ions in the whole-

Composition of mineral phases

Biotite

Ref. A8 V4 V l l V12 R3 R5 R12 R14

CaO (wt.%) Na 20 K20 10.4 8.57 9.54 7.70 8.15 8.94 9.14

Rb (ppm) 703 675 720 618 785 845 865 Sr 37 6 10 11 2 4 4 Ba 1,920 725 1,000 990 500 465 445 La 7.26 46.4 41.1 Ce 15.2 92.0 82.9 Nd 7.49 41.2 38.5 Sm 1.73 7.64 6.93 Eu 0.18 0.28 0.13 Tb 0.13 0.60 0.56 Yb 0.43 0.98 1.52 Lu 0.06 0.14 0.20 Hf 1.2 4.6 3.3 Th 1.9 24 28 Sc 41 54 53 61 63 58 V 795 320 343 343 246 240 242 Cr 384 215 241 245 150 135 145 145 Co 68 41 44 43 43 40 41 41 Ni 112 66 76 73 70 66 65 Cu 15 12 5 10 17 7 21 Zn 275 565 555 525 525 490 480

279

rock c o m p o s i t i o n . F u r t h e r m o r e , the co- ef f ic ients o f d i s t r ibu t ion o f R E E b e t w e e n b io t i t e and g r o u n d m a s s are re la t ively high in R o c c a s t r a d a (Table V) and decrease f r o m La to Yb with values s imilar to those r e p o r t e d b y Hi ld re th (1977) . On the o t h e r hand , these values f r o m Mt. A m i a t a are lower and increase f r o m La to Yb. T h e y r e semble the pa r t i t i on coef f ic ien ts f r o m daci tes (Schne tz le r and Phi lpot t s , 1970) . Such large var ia t ions typ ica l ly e n c o u n t e r e d in acidic rocks ( M a h o o d and Hi ldre th , 1983) re f lec t the inf luence o f the var ious pa r ame te r s , par t icu- larly o f l iquid c o m p o s i t i o n , on the pa r t i t ion coe f f i c i en t values a l though the p resence o f sub-mic roscop ic inclusions o f accessory minera ls c a n n o t be disregarded.

4.4. Frac t i ona l c rys ta l l i za t ion m o d e l

Quan t i t a t i ve t r ace -e l emen t mode l l ing o f f rac t iona l c rys ta l l iza t ion in acidic m a g m a is h inde red by the u n c e r t a i n t y o f the pa r t i t ion coef f i c ien t values and by the e f fec t s of accessory minera ls on the d i s t r ibu t ion o f several t race e lements . To o v e r c o m e these diff icult ies , the residual cha rac te r of the g r o u n d m a s s was tes ted using on ly t race e l emen t s (Ba, Rb, Sr) which are concen- t r a t ed in the pr inc ipa l minera l phases and fo r which e x p e r i m e n t a l pa r t i t ion coe f f i c i en t da ta are available.

The p r o p o r t i o n s of phases and residual l iquid were es tabl i shed f r o m the c o m p o s i t i o n s of minera ls (Dupuy , 1970; Balducci and

Sanidine Plagioclase Cordierite

AI O A25 V l l V l l R3 V1 R5 R14

Orthopyroxene

A1

0.72 0.75 0.41 8.32 2.26 2.24 2.75 6.34

12.3:[ 12.56 11.94 0.75

425 322 963 481

2,510 2,800 9.67 13.3 10.5 33.2

14.8 22.4 18.9 55.6 17.6

0.39 0.96 1.57 3.22 4.00 4.41 2.70 3.52 0.05 0.11 0.13 0.20 0.05 0.20 0.25 0.35 0.0~ 0.03 0.03 0.04 0.3 0.9 1 1.4 1.5 2.9 3.6 4.4

7.40 6.85 0.79

350 68

24.1 14.9 10.7 51.4 32.1 23.3 20.8 13.2 10.2

5.52 3.21 2.28 0.12 0.08 0.03 0.30 0.35 0.22 0.95 1.46 0.80 0.12 0.21 0.12 3.5 2.1 1.4

12 11 7.5 2 3 1

11 17 11 12 18 12 10

8 5

191

10 13 20

146 99 59 43

6 480

280

TABLE V

Partition coefficients (solid/liquid)

Biotite Cordierite Plagio- Ortho- Sani- clase pyrox- dine

ene

A8 R3 R5 V l l V1 R5 R14 V l l A1 AIO

Rb (ppm) 1.4 1.3 1.9 Sr 0.4 0.1 0.03 Ba 14.8 8.3 2.9 La 0.08 1.86 2.67 0.82 1.10 0.78 Ce 0.09 2.20 3.25 0.81 1.16 1.01 Nd 0.12 2.18 3.16 0.79 0.98 0.86 Sm 0.13 1.39 1.72 0.75 1.09 0.63 Eu 0.37 0.64 0.63 0.24 0.23 0.40 Tb 0.12 0.61 1.06 0.57 0.47 0.43 Yb 0.11 0.52 1.45 0.47 0.76 1.17 Lu 0.13 0.50 1.60 0.61 0.89 1.48 Hf 0.3 1.1 1.9 1.0 0.9 1.5 Th 0.1 1.7 2.6 1.0 1.1 2.6 Sc 14 16 16 12 0.5 0.6 V 132 12 13 Cr 64 25 21 22 2 2 Co 68 43 42 10 18 12 Ni 5 4.2 Zn 19 11 7.2

0.3 5.5 2.0

10 0.6 4

0.6 0.5 0.34 0.31 3.0 0.2 0.2 0.2

0.8 7.2

16 0.11 0.08

0.03 6.56 0.05 0.02 0.01 0.1 0.1

30 14 59 16

L e o n i , 1 9 8 1 ; V a n Be rgen , 1 9 8 4 ) , g r o u n d m a s s

and w h o l e - r o c k s us ing a l e a s t - s q u a r e s m i x i n g

e q u a t i o n . F o r e x a m p l e , t h e va lues o b t a i n e d

f o r t h r e e v o l c a n i c r o c k s f r o m Mt. A m i a t a

are g iven in T a b l e VI . T h e c a l c u l a t e d r e su l t s

are w i t h i n t h e r a n g e o f t h e a c t u a l m o d a l

c o m p o s i t i o n o f t h e rocks .

Fig. 7. Variations of Ba. vs. Rb in volcanic rocks of Tuscany and their groundmasses. Symbo l s are the same as in Fig. 2. Lines (1) and (2) = variations of Ba and Rb in residual liquids after fractional crystallization, line (1) = parent -- 392 ppm Rb and 555 ppm Ba,

proportions of crystallizing phases: 0.16 Bio, 0.42 Plg, 0.32 K-feld; line (2) = parent -- 420 ppm Rb and 140 ppm Ba, proportions of crystallizing phases: 0.10 Bio, 0.21 Plg, 0.34 K-feld, 0.35 Qtz. Numbers refer to the amount of residual liquid. Partition coeffi- cients used are from Drake and Weill (1975) and Long (1978) for plagioclase and K-feld- spar and from Crecraft et al. (1981) for biotite.

6 0 0

4 0 0

2 0 0

100

80

60

Ba ppm

+ • "'i.8t + .8

A . . . . . . . ( 1 1

.7-

' i

t

300

\ , , ~ , Rb ppm

500 700

281

TABLE VI

Mineral chemistry

(A) Composition of minerals used in calculation of the phase proportions in the residue

Bio Grt Cord Plg Co

SiO2 35.0 37.0 48.0 58.0 65.2 TiO2 5.0 0.8 A12 03 16.0 21.0 33.0 26.0 16.8 FeO T 21.0 30.0 10.0 5.0 MgO 8.5 5.0 7.0 2.4 CaO 1.0 9.0 1.4 Na20 0.3 6.0 2.4 K20 9.0 2.6

Co = average composition of the metapelitic rocks of the mica-schist group (Gianelli and Puxeddu, 1979); mineral composition after Van Bergen (1984).

(B) Calculated modal composition of the residue

Qtz Bio Cord Plg Grt F

Roccastrada 0.09 0.14 0.22 0.10 0.45 S. Vincenzo 0.25 0.05 0.20 0.05 0.65

F = degree of partial melting.

(C) Modal composition of Mt. Amiata ignimbrites

Plg Sand Bio Px Grdm

A1 0.151 0.125 0.059 0.045 0.619 A8 0.172 0.130 0.064 0.044 0.589 A10 0.162 0.073 0.067 0.043 0.654 x 0.14--0.17 0.10--0.16 0.02--0.04 0.03--0.04 0.06--0.66

A1, AS, and AIO = calculated compositions; x = range of modal compositions obtained by point-counting; Grdm = groundmass.

The mode l calculat ions of f ract ional crysta l l izat ion are shown toge the r with the whole - rock and g roundmass analyses for Rb and Ba in Fig. 7. The groundmasses fall on the calculated t rends close to the pred ic ted values based u p o n the degree o f solidif ication calculated by the least-squares me thod .

For samples, V1 and V l l f rom S. Vincenzo and A 2 5 f rom Mt. Amiata , the tie-lines connec t ing whole- rock and g roundmass have a less steep t rend (Fig. 7), which may be

accoun ted for by a smaller p r o p o r t i o n of sanidine a m o n g f rac t iona t ing phases com- pared to the o ther samples. The smaller a m o u n t o f sanidine is consis tent with the moda l compos i t i on o f the rocks. With the excep t ion o f the g roundmass of sample V12 , all o the r groundmasses represent residual liquids af ter crysta l l izat ion o f the present phenoc rys t s (Dupuy , 1970; D u p u y and All~gre, 1972). The g roundmass of V 1 2 is enr iched in Rb, Ba and Sr bu t has a major-

282

element composition similar to the whole- rock, indicating a more complex petrogenetic history.

Granodiorites from Mr. Capanne fall on the extension of the fractional crystallization trend of the rocks from Roccastrada (Fig. 7), suggesting that the volcanic rocks could have been formed by fractionation of a liquid with the composition of the granodiorite. However, such a relationship is not readily consistent with the Sr isotope data. The s7 Sr/ S6Sr initial ratio of the granodiorite ranges between 0.711 and 0.715 while the Roccastrada volcanic rocks have significantly higher values (> 0.719). Likewise the variations of Zr and HREE argue against the fractional crystallization model. The variation trends of the Roccastrada rocks seem to reflect the variable proportions of phenocrysts and groundmass. In fact, considering the high viscosity and yield strength of acidic magma (McBirney and Noyes, 1979), it is not expected that minerals of differing densities would separate from a melt rich in SiO2. In conclusion, the fractional crystallization process which can explain the differences in composition between whole-rocks and groundmasses, is probably operating in a closed system with continuous rehomogenization between liquid and crystals and its influence on the trace- element fractionation in the whole-rocks is relatively small.

5. Partial melting and origin of volcanic rocks from Tuscany

The partial melting of metasedimentary rocks has frequently been invoked to explain the origin of peraluminous acidic rocks (Chappell and White, 1974; Green, 1976; Clemens and Wall, 1981; Reid, 1983). This model has also been suggested for Tuscany acidic volcanic rocks by Marinelli (1961) and later documented by trace elements (Dupuy and All~gre, 1972) and isotope data (H.P. Taylor and Turi, 1976). H.P.

Taylor and Turi (1976) have considered the source of these rocks to be pelitic. The experimental data of Green (1976) show that the hydrous partial melting of pelitic material can produce, under certain P and T conditions, liquids which have compositions similar to the volcanic rocks of Roccastrada ( P = 4 k b a r , T = 7 8 0 ° C ) , S. Vincenzo ( P = 10 kbar, T = 820°C) and Mt. Amiata (P = 10 kbar, T = 1040 ° C). The STSr/86Sr isotope data are consistent with this model as far as the acidic volcanic rocks of Roccastrada and S. Vincenzo are concerned. However, the rocks of Mr. Amiata have a more complex origin. Several recent studies suggested for the Tuscany volcanics a mixing between crustal and mantle components (Hawkesworth and Vollmer, 1979; Poli et al., 1984; Van Bergen, 1984). This is also shown on Fig. 8 where the initial s7 Sr/S6 Sr ratio is plotted against 1/Sr using previously pub- lished values in addition to our data. Fig. 8 suggests that a mantle component was also involved in the genesis of the granodiorites and group II rocks of S. Vincenzo.

0725

0.720

0 715

0.710

. 87S r / / 865 r •

v i a

v1~ A +*

v 12A +•t + t ~ v t l

f l 1 2 f l 3 I I

~'/Sr~lO00

5 10 15 20

Fig. 8. Variations of 8~Sr/g6Sr ratio vs. 1/Sr in the Tuscan volcanics (symbols are the same as in Fig. 2) and K-rich basalts (stars) of the Roman province (Hawkesworth and Vollmer, 1979; Rogers et al., 1985). For the Tuscan volcanics, the data are from Hawkesworth and Vollmer (1979), Juteau et al. (1984), Poli et al. (1984) and this study. Symbols with reference hum hers are our data.

5.1. l gn im brites o f San V i ncenz o and R oc cas trada

The Paleozoic basement of Tuscany partly made up of metapelitic rocks might have been a source for the volcanic rocks of S. Vincenzo and Roccastrada. In order to test this hypothesis, the equilibrium partial melting equation of D.M. Shaw (1970) was employed assuming the source-rock com- position is similar to that of the Paleozoic mica-schist group from Tuscany given by Gianelli and Puxeddu (1979). The degree of partial melting and proportions of residual phases have been calculated (Table VI, using the least-squares equations, from the major- element composition of mica schists and a residual mineral assemblage the composition of which is given in Table VI. The results indicate a degree of partial melting (F) of 0.45 for the volcanic rocks of Roccastrada and 0.65 for those of S. Vincenzo. The calculated residual mineral proportions (Table VI) are in agreement with the experimental studies of Green (1976) and Winkler (1979) which suggest that partial melting of pelitic rocks produces a residue dominated by cordierite in association with quartz, biotite and plagioclase. In addition, garnet may also appear under high-P conditions. Several geochemical features of the rocks of Roccastrada and S. Vincenzo suggest the presence of some of these minerals in the residue. The depletion of transition elements imply the occurrence of a residual Fe--Mg mineral while a negative Eu anomaly is indicative of residual feldspars. A depletion of HREE in the volcanic rocks of S. Vincenzo points to garnet in their residue.

The bulk partition coefficients were determined from the proportions of mineral phases and the partition coefficients for biotite, K-feldspar and cordierite given in Table V. In addition, the partition coefficents for plagioclase were taken from Dudas et al. (1971) and for garnet from Irving and Frey (1978). The model calculations (Fig. 9) show that, with the exception of Rb, the partial

283

0.5

0.2

5 a: 5 2_

2 C/)

1 0

0.5

0.2

" \ \ \

J i i i i i

i i k I i i , i i i ,

Rb Y b T b S m Ce La Eu Z n Sc Sr Ba Co V

Fig. 9. Ranges of t r ace -e l emen t a b u n d a n c e s in acidic volcanic rocks of Roccas t r ada (A) and S. V incenzo (B) no rma l i zed to the u p p e r c o n t i n e n t a l c rus t (Tay lo r and M c L e n n a n , 1981) (solid lines). The dashed lines r ep resen t the range of l iquid c o m p o s i t i o n s p r o d u c e d by par t ia l me l t i ng of m e t a s e d i m e n t a r y rocks. The source c o m p o s i t i o n ( in p p m ) : Rb = 128 ± 36, Sr = 180 + 43, B a = 5 3 6 ± 170, S c = 10± 2, V = 1 0 9 - + 50, Co = 15 ± 5, Zn = 8 5 ± 30, La = 3 0 ± 3, C e = 6 4 ± 6, Sm = 4.5 -+ 0.4, Eu = 0 .88 ± 0.09, T b = 0 . 6 4 ± 0.06, Yb = 2.2 ± 0.2. The values for Rb , Sr, Ba, V, Co and Zn are the averages of the mica-schis t group of Tuscany (Gianel l i and Puxeddu , 1979) . Those for Sc and REE are the averages of the u p p e r c rus t (S.R. Tay lo r and M c L e n n a n , 1 9 8 1 ) ± 10%. The pa r t i t i on coef f ic ien ts for b io t i t e and cord ie r i t e are f rom Table V whereas those of plagioclase are f rom Dudas et al. ( 1 9 7 1 ) and ga rne t f rom Irving and Frey (1978) . Pro- p o r t i o n s of phases in the residue are: Roccas t r ada : 0 .40 Bio, 0.41 Cord, 0 .19 Plg; and San Vincenzo : 0 .26 Bio, 0 .24 Grt , 0 .50 Cord.

melting of the mica schists can account for the composition of the volcanic rocks provided that the Ba partition coefficient of biotite is ~ 15 and that of plagioclase for Sr is in the range 10--15. In addition, the volcanic rocks of Roccastrada require either the presence of a small amount of K-feldspar in the residue or a source slightly depleted in Ba and Sr compared to the mica schists. The results also suggest that all the elements involved in

284

the calculat ions, excep t Rb, behave as in- Mt. Amia ta could have been genera ted by compa t ib l e e lements dur ing the partial partial mel t ing o f metapel i t ic rocks. The mel t ing o f the crust. This applies even to similarities in the concen t ra t ions o f t ransi t ion e lements such as REE which have bulk elements, Zr, Hf, Sr and Ba be tween the par t i t ion coeff ic ients a round 1. volcanic rocks and presumed source imply a

near ly to ta l melting. However , such a high 5 2. Volcanic rocks of Monte Amiata degree o f melt ing c a n n o t explain the enrich-

m e n t o f K, Rb, Pb, Li and REE in the Accord ing to the major -e lement volcanic rocks. Alternat ively, the exper-

compos i t ions , the acidic volcanic rocks of imenta l studies o f Green and R ingwood

TABLE VII

Mixing proportions of crust and upper-mantle components calculated from isotopic Sr ratios and content

Component a7 Sr/86 Sr Sr (ppm)

Proportion of mixing

granodiorite, ignimbrite, trachyte, Mt. Capanne Mt. Amiata Mt. Amiata

Crust 0.722 80 0.90--0.95 0.80--0.85 0.68--0.73 Upper mantle 0.711 1700 0.10--0.05 0.20--0.15 0.32--0.27

Crust = average of volcanic rocks of Roccastrada and S. Vincenzo (group I); upper mantle = K-rich basalts of the Roman magmatic province (Hawkesworth and Vollmer, 1979; Rogers et al., 1985).

TABLE VIII

Mixing proportions of crust and upper-mantle components for trace elements

1 2 3 4 5 6

Rb(ppm) 420(35) 497(176) 392 (2) 392 (2) 359(11) 357(11) Sr 60 (11) 1600 (175) 385 (9) 382 (9) 546 (50) 490(80) Ba 140 (36) 1580 (383) 552 (19) 555 (20) 625 (47) 625 (50) La 30 (4) 139(17) 66 (2) 69 (3) 71 (1) 71 (1) Ce 63 (8) 243(33) 131 (5) 138 (6) 143 (1) 143 (1) Nd 29 (4) 110(12) 55 (2) 57 (2) 62 (1) 62 (1) Y 30 (2) 32 (5) 33 (1) 33 (1) 33 (1) 33 (1) Zr 103 (17) 378 (74) 211(11) 225 (12) 249 (2) 249 (2) Nb 14 (1) 31 (6) 17 (1) 16 (1) 16 (1) 15 (1) Th 23 (3) 83(24) 46 (1) 46 (1) 42 (3) 42 (3) Pb 43 (3) 99(35) 59 (1) 59 (1) 59 (6) 58 (7) U 16 (2) 15 (6) 13 (1) 11 (2) 12 (1) 10 (2)

Proportion (I) 0.76 (0.01) 0.66 (0.02) of components (II) 0.24 (0.01) 0.33 (0.02)

1 = crustal component (I): average of Roccastrada volcanic rocks (this study); 2 = mantle component (II): leucitic tephrite of Roman province (Holm et al., 1982); 3, 4 = calculated content and average, respectively, of ignimbrites from Mt. Amiata (cf. Table VII); 5. 6 = calculated content and average, respectively, of trachyte lava from Mt. Amiata (cf. Table VII); values in brackets = standard deviation.

285

(1968) have shown that 30--40% of melting of granulite or amphiboli te may produce a liquid with SiO2 content similar to the rocks of Mt. Amiata. Although this process is consistent with the abundances of transition elements, it cannot account for the high contents of many incompatible elements which require a very low degree of melting. Thus it appears that a simple partial melting model cannot explain the composi t ion of volcanic rocks from Mt. Amiata.

In Fig. 8, the volcanic rocks of Mr. Amiata, S. Vincenzo (group II) and granodiorite of Mt. Capanne plot along a line, suggesting a mixing between two components . Van Bergen (1984) suggested that the mantle component corresponds to the K-rich basalts of the Roman magmatic province. In order to evaluate the proport ions of the mantle componen t a mixing model was calculated assuming that the crustal component has the compost ion of the average of the volcanic rocks from Roccastrada and S. Vincenzo (group I). The results reported in Table VII indicate that ignimbrites of Mr. Amiata have incorporated 15--20% of upper-mantle com- ponent. In trachyte lavas, the amount increases to 27--32% while the granodiorite of Mt. Capanne has only 5--10% of the upper- mantle component .

Similar calculations for trace elements (Table VIII) indicate that the composit ion of the various volcanics of Mt. Amiata may be explained by the mixing between leucitic tephrite of the Roman magmatic province (Holm et al., 1982) and a crustal componen t with the composi t ion of the acidic volcanic rocks of Roccastrada. The mixing of these two components, in proport ions obtained from trace elements, can also account for the major-element composit ions of the rocks.

6. Conclusions

The acidic volcanic rocks of Tuscany are characterized by a distinct enrichment of K, Rb, Li, Pb, U and Th and show considerable composit ional differences among various

volcanic centres. These features reflect the complexities of the processes involved in their genesis.

The volcanic rocks of Roccastrada and S. Vincenzo (group I) were probably generated by partial melting of metapelitic rocks which form a part of the Paleozoic basement of Tuscany, leaving a residue composed of biotite, quartz and cordierite. In addition, the residue of the Roccastrada volcanic rocks probably also contains plagioclase whereas that of the S. Vincenzo rocks include garnet. However, this mechanism cannot account for the enrichment of K, Rb, Li, Pb, U and Th, which suggests either an anomalously enriched source or a secondary process of enrichment. In fact, the selective contamination by fluids derived from the subducted slab and rising into the overlying mantle wedge was invoked for the basalts of the Roman magmatic province (Cox et al., 1976; Hawkesworth and Vollmer, 1979; Vollmer and Hawkesworth, 1980) and may have also affected the overlying crust. In addition, recent geophysical data provide evidence for the presence of two crust-- mantle transitions in Tuscany, one at a depth of 25--30 km and the other at ~ 60 km depth (Morelli et al., 1977). Thus it may be sugges- ted that the fluids which contaminated the source of the Roccastrada and S. Vincenzo acidic volcanic rocks were derived from the deeper crust.

The other acidic volcanic rocks of Tuscany have been additionally affected by contami- nation or mixing with an upper-mantle component . This component might have been K-rich basalts of the Roman magmatic province which also have a high initial Sr isotope ratio (Rogers et al., 1985). The addition of this component can explain the geochemical characteristics of the volcanic rocks, including their major- and trace- element composit ion. Its influence is most distinct in the rocks from Mt. Amiata.

The model suggested for the origin of the Tuscan acidic volcanic rocks can be applied to the granodiorites of Mt. Caponne. The

286

p lu ton is p r e d o m i n a n t l y the p r o d u c t o f upper -c rus t a l anatexis . Howeve r , the m e l t was c o n t a m i n a t e d and con ta ins 5 - -10% of a c o m p o n e n t wi th geochemica l charac te r i s t ics o f the K-rich basalts. The d is t inc t s imilari t ies in i so tope and t r a ce - e l em en t c o m p o s i t i o n b e t w e e n the g ranod ior i t e s and the S. Vincenzo volcanic rocks o f g roup I I indica te tha t the s low cool ing o f the g ranod io r i t e has no t a f f ec t ed the d i s t r ibu t ion o f t race e l emen t s wi th the e x c e p t i o n of Li. The high c o n t e n t and d i s t r ibu t ion of this e l e m e n t in the volcanic rocks m a y be due to a late i n t e rac t ion wi th the su r round ing rocks .

The process o f f rac t iona l c rys ta l l i za t ion in the Tuscan acidic suites has t aken place in a c losed sy s t em and can a c c o u n t fo r the d i f fe rences in c o m p o s i t i o n b e t w e e n whole- rocks and groundmass /g lass . However , the r a the r l imi ted range o f t r a c e - e l e m e n t va r ia t ions wi th in each m a g m a t i c cen t re p r o b a b l y ref lects on ly var ia t ion in the relat ive p r o p o r t i o n s of g roundmass /g lass and phenoc rys t s .

The resul ts show tha t the high degree o f par t ia l me l t ing o f the crust p r o d u c e s l iquids wi th c o n c e n t r a t i o n s o f m o s t t race e l emen t s c o m p a r a b l e to those o f the source rocks . The e x c e p t i o n is the m o s t c o m p a t i b l e e l emen t s ( t rans i t ion e lements , Ba and Sr) which m a y be pa r t ly r e ta ined in the residue. The high c o n t e n t s o f L R E E , Ba, Sr and some o the r i n c o m p a t i b l e e l em en t s in rocks such as the volcanics o f Mt. A m i a t a imp ly a s e c o n d a r y process o f en r i chmen t . F o r the acidic rocks o f Tuscany , the m o s t plausible process of e n r i c h m e n t is i n t e r ac t ion wi th an u p p e r - m a n t l e c o m p o n e n t .

The var ia t ion t r ends o f t race e l em en t s obse rved in the Tuscan volcanic rocks are similar to those r e p o r t e d f r o m m a n y acidic p lu tons (e.g., S.E. Shaw and F lood , 1981) , suggest ing tha t the pe t rogene t i c m o d e l fo r Tuscan rocks has some general impl ica t ions . In fact , a process o f crusta l ana tex i s a c c o m p a n i e d b y in t e r ac t ion wi th u p p e r - m a n t l e c o m p o n e n t s has been f r e q u e n t l y p o s t u l a t e d in the genesis o f grani tes (All~gre, 1982) .

A c k n o w l e d g e m e n t s

The s t udy was s u p p o r t e d by CGG of Montpe l l i e r (F rance) and N S E R C of Canada (ope ra t ing grant A 3872) . T h a n k s are due to Dr. P. Vidal fo r i so top ic de t e rmina t ions .

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